Retroreflecting optical construction

ABSTRACT

Retroreflecting optical constructions are disclosed. A disclosed retroreflecting optical construction includes a retroreflecting layer that has a retroreflecting structured major surface, and an optical film that is disposed on the retroreflecting structured major surface of the retroreflecting layer. The optical film has an optical haze that is not less than about 30%. Substantial portions of each two neighboring major surfaces in the retroreflecting optical construction are in physical contact with each other.

RELATED APPLICATIONS

This application claim priority from U.S. Provisional Patent ApplicationSer. No. 61/169,532, filed Apr. 15, 2009, the disclosure of which isherein incorporated by reference in its entirety.

This application is related to the following U.S. Patent Applications,filed on even date herewith and which are incorporated by reference:U.S. Application No. 61/169,466, entitled “Optical Film”; U.S.Application No. 61/169,521, entitled “Optical Construction and DisplaySystem Incorporating Same”; U.S. Application No. 61/169,549, entitled“Optical Film for Preventing Optical Coupling”; U.S. Application No.61/169,555, entitled “Backlight and Display System Incorporating Same”;U.S. Application No. 61/169,427, entitled “Process and Apparatus forCoating with Reduced Defects”; and U.S. Application No. 61/169,429,entitled “Process and Apparatus for Ananovoided Article”.

FIELD OF THE INVENTION

This invention generally relates to retroreflective optical articlesthat include a low index porous optical film. The invention is furtherapplicable to retroreflective optical articles that include an opticallydiffusive film that exhibit some low-index-like properties.

BACKGROUND

Retroreflective sheetings reflect incident light back toward theoriginating light source. Retroreflective sheetings are commonly usedin, for example, road signs, license plates, barricades and safetygarments to improve or enhance their visibility in poor lightingconditions.

Cube corners are commonly used in retroreflective sheetings. Typically,a cube corner includes three mutually perpendicular optical faces thatintersect at a single apex. Generally, light that is incident on acorner cube from a light source, is totally internally reflected fromeach of the three perpendicular cube corner optical faces, and isredirected back toward the light source. Presence of, for example, dirt,water and adhesive on the optical faces can prevent total internalreflection (TIR) and lead to a reduction in the retroreflected lightintensity. As such, the air interface is typically protected by asealing film, but such films typically reduce the total active area,which is the area over which retroreflection can occur. Metallized cubecorners do not rely on TIR for retroreflecting light, but they aretypically not white enough for daytime viewing of, for example, signingapplications. Furthermore, the durability of the metal coatings may alsobe inadequate.

SUMMARY OF THE INVENTION

Generally, the present invention relates to retroreflecting opticalconstructions. In one embodiment, an optical construction includes aretroreflecting layer that has a retroreflecting structured majorsurface, an optical film that is disposed on the retroreflectingstructured major surface and has an effective index of refraction thatis not greater than about 1.3, and an optically diffusive layer that isdisposed on the optical film and has an optical haze that is not lessthan about 30%. Substantial portions of each two neighboring majorsurfaces in the retroreflecting optical construction are in physicalcontact with each other. In some cases, the optical film has aneffective index of refraction that is not greater than about 1.2, or notgreater than about 1.15, or not greater than about 1.1. In some cases,the optically diffusive layer has an optical haze that is not less thanabout 50%, or not less than about 70%, or not less than about 90%. Insome cases, at least 50%, or at least 70%, or at least 90%, of each twoneighboring major surfaces in the retroreflecting optical constructionare in physical contact with each other. In some cases, the optical filmsubstantially planarizes the retroreflecting layer. In some cases, theoptical film includes a binder, a plurality of particles and a pluralityof interconnected voids, where a volume fraction of the plurality ofinterconnected voids in the optical film is not less than about 20% andthe weight ratio of the binder to the plurality of the particles is notless than about 1:1.

In another embodiment, a retroreflecting optical construction includes aretroreflecting layer that has a retroreflecting structured majorsurface, and an optical film that is disposed on the retroreflectingstructured major surface of the retroreflecting layer and has an opticalhaze that is not less than about 30%. Substantial portions of each twoneighboring major surfaces in the retroreflecting optical constructionare in physical contact with each other. In some cases, at least 50%, orat least 70%, or at least 90%, of each two neighboring major surfaces inthe retroreflecting optical construction are in physical contact witheach other. In some cases, the optical film includes a plurality ofparticles and a plurality of interconnected voids, where the volumefraction of the plurality of interconnected voids in the optical film isnot less than about 20% and the weight ratio of the plurality of theparticles to the binder is in a range from about 2:1 to about 6:1.

In another embodiment, a retroreflecting optical construction includes aretroreflecting layer that has a retroreflecting structured majorsurface, and an optical film that is disposed on a first portion of theretroreflecting structured major surface. The optical film includes abinder, a plurality of particles and a plurality of voids. The firstportion of the retroreflecting structured major surface exhibits acoefficient of retroreflection R_(A) that is not less than about 50cd/(lux·m²) for an observation angle of 0.2 degrees and an entranceangle of −4 degrees. In some cases, the first portion is not less thanabout 30%, or not less than about 45%, or not less than about 60%, ofthe retroreflecting structured major surface. In some cases, RA is notless than about 100 cd/(lux·m²), or not less than about 200 cd/(lux·m²),or not less than about 300 cd/(lux·m²), for an observation angle of 0.2degrees and an entrance angle of −4 degrees.

In another embodiment, a retroreflecting optical construction includes aretroreflecting layer that has a retroreflecting structured majorsurface, and an optical film that is disposed on a first portion of theretroreflecting structured major surface. The optical film includes abinder, a plurality of particles and a plurality of voids. The firstportion of the retroreflecting structured major surface exhibits a totallight return that is not less than about 5% for incident visible lightat an entrance angle of −4 degrees. In some cases, the total lightreturn is not less than about 10%, or not less than about 20%, or notless than about 30%, for incident visible light at an entrance angle of−4 degrees.

BRIEF DESCRIPTION OF DRAWINGS

The invention may be more completely understood and appreciated inconsideration of the following detailed description of variousembodiments of the invention in connection with the accompanyingdrawings, in which:

FIG. 1A is a schematic side-view of a retroreflecting opticalconstruction;

FIG. 1B is a schematic top-view of a retroreflecting layer;

FIG. 2 is a schematic side-view of another retroreflecting opticalconstruction;

FIG. 3 is a schematic side-view of another retroreflecting opticalconstruction;

FIG. 4 is a schematic side-view of another retroreflecting opticalconstruction;

FIG. 5 is a schematic side-view of another retroreflecting opticalconstruction;

FIG. 6 is a schematic side-view of another retroreflecting opticalconstruction;

FIGS. 7A and 7B are respective schematic top-view and side-view of anindividual prism in a prismatic retroreflecting layer; and

FIGS. 8A and 8B are respective schematic top-view and side-view of anindividual prism in another prismatic retroreflecting layer.

In the specification, a same reference numeral used in multiple figuresrefers to the same or similar elements having the same or similarproperties and functionalities.

DETAILED DESCRIPTION

This invention generally relates to retroreflective films and opticalconstructions that include one or more optical films that have a lowindex of refraction or exhibit some low-index-like optical properties.Some disclosed retroreflective optical constructions include one or moreoptical films that have a low optical haze or diffuse reflectance and alow effective index of refraction, such as an optical haze of less thanabout 5% and an effective index of refraction that is less than about1.3. In such cases, the low index optical films can efficiently supportor maintain total internal reflection. Some disclosed retroreflectiveoptical constructions include one or more optical films that have a highoptical haze or diffuse reflectance while manifesting somelow-index-like optical properties, such as, for example, the ability tosupport total internal reflection or enhance internal reflection. Aretroreflective optical construction that incorporates such a diffusiveoptical film can have a white appearance, a desirable attributeespecially in daylight viewing, while at the same time, retroreflectingwith high efficiency.

Some disclosed optical films support total internal reflection (TIR) orenhanced internal reflection (EIR) by virtue of including a plurality ofvoids. When light that travels in an optically clear non-porous mediumis incident on a stratum possessing high porosity, the reflectivity ofthe incident light is much higher at oblique angles than at normalincidence. In the case of no or low haze voided films, the reflectivityat oblique angles greater than the critical angle is close to about100%. In such cases, the incident light undergoes total internalreflection (TIR). In the case of high haze voided films, the obliqueangle reflectivity can be close to 100% over a similar range of incidentangles even though the light may not undergo TIR. This enhancedreflectivity for high haze films is similar to TIR and is designated asEnhanced Internal Reflectivity (EIR). As used herein, by a porous orvoided optical film enhancing internal reflection (EIR), it is meantthat the reflectance at the boundary of the voided and non-voided strataof the film or film laminate is greater with the voids than without thevoids.

The disclosed optical films include a plurality of voids dispersed in abinder. The voids have an index of refraction n_(v) and a permittivityε_(v), where n_(v) ²=ε_(v), and the binder has an index of refractionn_(b) and a permittivity ε_(b), where n_(b) ²=ε_(b). In general, theinteraction of an optical film with light, such as light that isincident on, or propagates in, the optical film, depends on a number offilm characteristics such as, for example, the film thickness, thebinder index, the void or pore index, the pore shape and size, thespatial distribution of the pores, and the wavelength of light. In somecases, light that is incident on or propagates within the optical film,“sees” or “experiences” an effective permittivity ε_(eff) and aneffective index n_(eff), where n_(eff) can be expressed in terms of thevoid index n_(v), the binder index n_(b), and the void porosity orvolume fraction “f”. In such cases, the optical film is sufficientlythick and the voids are sufficiently small so that light cannot resolvethe shape and features of a single or isolated void. In such cases, thesize of at least a majority of the voids, such as at least 60% or 70% or80% or 90% of the voids, is not greater than about λ/5, or not greaterthan about λ/6, or not greater than about λ/8, or not greater than aboutλ/10, or not greater than about λ/20, where λ is the wavelength oflight.

In some cases, light that is incident on a disclosed optical film is avisible light meaning that the wavelength of the light is in the visiblerange of the electromagnetic spectrum. In such cases, the visible lighthas a wavelength that is in a range from about 380 nm to about 750 nm,or from about 400 nm to about 700 nm, or from about 420 nm to about 680nm. In such cases, the optical film has an effective index of refractionand includes a plurality of voids if the size of at least a majority ofthe voids, such as at least 60% or 70% or 80% or 90% of the voids, isnot greater than about 70 nm, or not greater than about 60 nm, or notgreater than about 50 nm, or not greater than about 40 nm, or notgreater than about 30 nm, or not greater than about 20 nm, or notgreater than about 10 nm.

In some cases, the disclosed optical films are sufficiently thick sothat the optical film can reasonably have an effective index that can beexpressed in terms of the indices of refraction of the voids and thebinder, and the void or pore volume fraction or porosity. In such cases,the thickness of the optical film is not less than about 100 nm, or notless than about 200 nm, or not less than about 500 nm, or not less thanabout 700 nm, or not less than about 1,000 nm.

When the voids in a disclosed optical film are sufficiently small andthe optical film is sufficiently thick, the optical film has aneffective permittivity E_(eff) that can be expressed as:ε_(eff) =fε _(v)+(1−f)ε_(b)  (1)

In such cases, the effective index n_(eff) of the optical film can beexpressed as:n _(eff) ² =fn _(v) ²+(1−f)n _(b) ²  (2)

In some cases, such as when the difference between the indices ofrefraction of the pores and the binder is sufficiently small, theeffective index of the optical film can be approximated by the followingexpression:n _(eff) =fn _(v)+(1−f)n _(b)  (3)

In such cases, the effective index of the optical film is the volumeweighted average of the indices of refraction of the voids and thebinder. For example, an optical film that has a void volume fraction ofabout 50% and a binder that has an index of refraction of about 1.5, hasan effective index of about 1.25.

FIG. 1A is a schematic side-view of a retroreflecting opticalconstruction 900 that includes a retroreflecting layer 930 that includesa front major surface 936 that faces a viewer 905 and a retroreflectingstructured major surface 940 opposite surface 936, an optical film 960disposed on retroreflecting major surface 940, an optical adhesive layer970 disposed on the optical film, an optically diffusive layer 995disposed on the optical adhesive layer, and a first substrate 980disposed on the optical adhesive layer. Retroreflecting opticalconstruction 900 further includes an optically transparent secondsubstrate 920 disposed on front major surface 936 of the retroreflectinglayer and a graphics layer 910 disposed on the second substrate.

The coefficient of retroreflection R_(A), sometimes referred to as theretroreflectivity, of retroreflecting optical construction 900 can varydepending on the properties desired in an application. In some cases,R_(A) meets the ASTM D4956-07e1 standards at 0 degree and 90 degreeorientation angles. In some cases, R_(A) is in a range from about 5cd/(lux·m²) to about 1500 cd/(lux·m²) when measured at 0.2 degreeobservation angle and +5 degree entrance angle according to ASTM E-810test method or CIE 54.2; 2001 test method. In some cases, such as incases where optical construction 900 is used in a traffic control sign,a delineator, or a barricade, R_(A) is at least about 330 cd/(lux·m²),or at least about 500 cd/(lux·m²), or at least about 700 cd/(lux·m²) asmeasured according to ASTM E-810 test method or CIE 54.2; 2001 testmethod at 0.2 degree observation angle and +5 degree entrance angle. Insome cases, such as in motor vehicle related application, R_(A) is atleast about 60 cd/(lux·m²), or at least about 80 cd/(lux·m²), or atleast about 100 cd/(lux·m²) as measured according to ASTM E-810 testmethod or CIE 54.2; 2001 test method at 0.2 degree observation angle and+5 degree entrance angle.

Retroreflecting layer 930 includes a retroreflecting portion 934 thatincludes a plurality or an array of retroreflecting optical elements 950and a land portion 932 that connects the retroreflecting opticalelements. FIG. 1B is a schematic top-view of retroreflecting layer 930.In some cases, such as in the exemplary optical construction 900, eachretroreflecting optical element 950 is in the form of a tetrahedron or apyramid, such as a regular tetrahedron or pyramid, having three planarfacets or sides 952 and a base 956, where the sides meet at an apex 954.The dihedral angle between any two facets may vary depending on theproperties desired in an application. In some cases, the dihedral anglebetween any two facets 952 is 90 degrees. In such cases, facets 952 aresubstantially perpendicular to one another (as in the corner of a room)and the optical element may be referred to as a cube corner. In somecases, the dihedral angle between adjacent facets 952 can deviate from90° as described, for example, in U.S. Pat. No. 4,775,219, thedisclosure of which is incorporated in its entirety herein by reference.

In some cases, optical elements 950 can be truncated cube corners. Insome cases, optical elements 950 can be full cubes or preferred geometry(PG) cubes as described in, for example, U.S. Pat. No. 7,422,334, thedisclosure of which is incorporated in its entirety herein by reference.

Each retroreflecting optical element 950 includes a symmetry axis 957that makes equal angles with facets 952. In some cases, such as in theexemplary optical construction 900, symmetry axis 957 is perpendicularto base 956 or front surface 936. In some cases, the symmetry axis isnot perpendicular to the base or the front surface. In such cases, apex954 or optical element 950 is canted as described, for example, in U.S.Pat. No. 4,588,258.

The principle of operation of a retroreflective cube corner is wellknown and is described, for example, in J. Optical Soc. of America46(7), 496 (1958). In sum, a light ray 990 propagating along thepositive y-direction and incident on a retroreflecting optical element950, is totally internally reflected (TIR) by each of facets 952 of theoptical element resulting in a retroreflected light ray 993 propagatingalong the negative y-direction and parallel to incident light ray 990.In some cases, the retroreflected light ray deviates from they-direction as retroreflected light ray 992 making a divergence angle δwith the y-axis. In some cases, such as in the case of a road sign, thedivergence angle δ is in a range from about 0.2 degrees to about 2degrees. Any breakdown of TIR can substantially reduce the intensity ofretroreflected light ray 993.

Optical film 960 has a sufficiently low index of refraction so as tomaintain or support TIR resulting in efficient retroreflection byretroreflective layer 930. In some cases, the effective index ofrefraction of optical film 960 is not greater than about 1.3, or notgreater than about 1.25, or not greater than about 1.2, or not greaterthan about 1.15, or not greater than about 1.1.

Optical adhesive layer 970 adheres optical film 960 to opticallydiffusive layer 995. In some cases, adhesive layer 970 is substantiallyoptically diffusive and can have a white appearance. For example, insuch cases, the optical haze of an optically diffusive adhesive layer970 is not less than about 30%, or not less than about 40%, or not lessthan about 50%, or not less than about 60%, or not less than about 70%,or not less than about 80%, or not less than about 90%, or not less thanabout 95%. In some case, the diffuse reflectance of the diffusiveadhesive layer is not less than about 20%, or not less than about 30%,or not less than about 40%, or not less than about 50%, or not less thanabout 60%. In such cases, the adhesive layer can be optically diffusiveby including a plurality of particles dispersed in an optical adhesivewhere the particles and the optical adhesive have different indices ofrefraction. The mismatch between the two indices of refraction canscatter light. In some cases, such as when optical adhesive layer 970 isoptically diffusive, optical construction 900 may not include theoptically diffusive layer 995.

Optical adhesive layer 970 can include any optical adhesive that may bedesirable and/or available in an application. Exemplary opticaladhesives include pressure sensitive adhesives (PSAs), heat-sensitiveadhesives, solvent-volatile adhesives, and UV-curable adhesives such asUV-curable optical adhesives available from Norland Products, Inc.Exemplary PSAs include those based on natural rubbers, syntheticrubbers, styrene block copolymers, (meth)acrylic block copolymers,polyvinyl ethers, polyolefins, and poly(meth)acrylates. As used herein,(meth)acrylic (or acrylate) refers to both acrylic and methacrylicspecies. Other exemplary PSAs include (meth)acrylates, rubbers,thermoplastic elastomers, silicones, urethanes, and combinationsthereof. In some cases, the PSA is based on a (meth)acrylic PSA or atleast one poly(meth)acrylate. Exemplary silicone PSAs include a polymeror gum and an optional tackifying resin. Other exemplary silicone PSAsinclude a polydiorganosiloxane polyoxamide and an optional tackifier.

In some cases, optical adhesive layer 970 can include cross-linkedtackified acrylic pressure sensitive adhesives. Optical adhesive layer970 can include additives such as tackifiers, plasticizers and fillers(such as pigments such as TiO₂). In some cases, TiO₂ can be added to theadhesive layer to give it a white appearance.

Optically diffusive layer 995 diffuses incident light and canadvantageously give a white appearance to optical construction 900 in,for example, daylight conditions. Optically diffusive layer 995 can beany optically diffusive layer that may be desirable and/or available inan application. For example, the optically diffusive layer can include aplurality of particles dispersed in a binder where the particles and thebinder have different indices of refraction. In some cases, such as whenoptically diffusive layer 995 is sufficiently diffusive to impart awhite look to optical construction 900, the optically diffusive layerhas an optical haze that is not less than about 40%, or not less thanabout 50%, or not less than about 60%, or not less than about 70%, ornot less than about 80%, or not less than about 90%, or not less thanabout 95%.

In some cases, optically diffusive layer 995 can also be an adhesive. Insuch cases, the optically diffusive layer 995 can provide sufficientadhesion, in which case, optical construction 900 may not includeoptical adhesive layer 970.

Substantial portions of neighboring major surfaces of each twoneighboring layers in optical construction 900 are in physical contactwith each other. For example, substantial portions of neighboringstructured major surfaces 951 and 940 of respective neighboring layers960 and 930 in optical construction 900 are in physical contact witheach other. For example, at least 30%, or at least 40%, or at least 50%,or at least 60%, or at least 70%, or at least 80%, or at least 90%, orat least 95% of the two neighboring major surfaces are in physicalcontact with each other. In some cases, optical film 960 is coated onsurface 940 of retroreflecting layer 930.

In general, substantial portions of neighboring major surfaces (majorsurfaces that face each other or are adjacent to each other) of each twoneighboring layers in optical construction 900 are in physical contactwith each other. For example, in some cases, there may be one or moreadditional layers, not expressly shown in FIG. 1A, disposed betweenoptical film 960 and retroreflecting layer 930. In such cases,substantial portions of neighboring major surfaces of each twoneighboring layers in optical construction 900 are in physical contactwith each other. In such cases, at least 30%, or at least 40%, or atleast 50%, or at least 60%, or at least 70%, or at least 80%, or atleast 90%, or at least 95% of the neighboring major surfaces of each twoneighboring layers in the optical construction are in physical contactwith each other.

Optical film 960 can be any optical film that has a sufficiently lowindex of refraction, such as those described in co-pending applicationtitled “OPTICAL FILM”, U.S. Application No. 61/169,466, , filed on thesame date as the present application, the disclosure of which isincorporated in its entirety herein by reference. In some cases, opticalfilm 960 includes a binder, a plurality of particles and a plurality ofinterconnected voids. The volume fraction of the plurality ofinterconnected voids in the optical film is not less than about 20%, ornot less than about 30%, or not less than about 40%, or not less thanabout 50%, or not less than about 60%, or not less than about 70%, ornot less than about 80%. The weight ratio of the binder to the pluralityof particles is not less than about 1:1, or not less than about 1.5:1,or not less than about 2:1, or not less than about 2.5:1, or not lessthan about 3:1, or not less than about 3.5:1, or not less than about4:1. The optical film has a substantially low optical haze. For example,in such cases, the optical haze of the optical film is not greater thanabout 10%, or not greater than about 7%, or not greater than about 5%,or not greater than about 3%, or not greater than about 2%, or notgreater than about 1.5%, or not greater than about 1%. In some cases,the particles in the optical film can be approximately spherical. Insome cases, the particles can be elongated.

FIG. 2 is a schematic side-view of a retroreflecting opticalconstruction 901 that includes optional graphics layer 910, secondsubstrate 920 disposed on the graphics layer, retroreflecting layer 930disposed on the second substrate, an optical film 965 disposed on theretroreflecting layer, optical adhesive layer 970 disposed on theoptical film, and first substrate 980 disposed on the optical adhesivelayer.

Optical film 965 is substantially optically diffusive. For example,optical film 965 has an optical haze that is not less than about 20%, ornot less than about 30%, or not less than about 40%, or not less thanabout 50%, or not less than about 60%, or not less than about 70%, ornot less than about 80%, or not less than about 90%, or not less thanabout 95%. In some cases, the diffuse reflectance of the optical film isnot less than about 20%, or not less than about 30%, or not less thanabout 40%, or not less than about 50%, or not less than about 60%.

Furthermore, optical film 965 exhibits some low-index-like properties.In particular, optical film 965 supports or maintains TIR and/orpromotes internal reflection at the interface with retroreflecting layer930.

Optical film 965 includes a structured major surface 966 that facesstructured major surface 940 of retroreflecting layer 930. Substantialportions of neighboring major surfaces of each two neighboring layers inoptical construction 901 are in physical contact with each other. Forexample, substantial portions of neighboring structured major surfaces966 and 940 respective neighboring layers 965 and 930 in opticalconstruction 901 are in physical contact with each other. For example,at least 50%, or at least 60%, or at least 70%, or at least 80%, or atleast 90%, or at least 95% of the two neighboring major surfaces are inphysical contact with each other. In some cases, optical film 965 iscoated on structured surface 940 of the retroreflecting layer.

In general, substantial portions of neighboring major surfaces (majorsurfaces that face each other or are adjacent to each other) of each twoneighboring layers in optical construction 901 are in physical contactwith each other. For example, in some cases, there may be one or moreadditional layers, not expressly shown in FIG. 2, disposed in betweenretroreflecting layer 930 and optical film 965. In such cases,substantial portions of neighboring major surfaces of each twoneighboring layers in optical construction 901 are in physical contactwith each other. In such cases, at least 50%, or at least 60%, or atleast 70%, or at least 80%, or at least 90%, or at least 95% of theneighboring major surfaces of each two neighboring layers in the opticalconstruction are in physical contact with each other.

In general, optical film 965 can be any optically diffusive film thatpromotes or maintains TIR or enhances internal reflection such as thosedescribed in co-pending application titled “OPTICAL FILM”, U.S.Application No. 61/169,466. In some cases, the optical film includes abinder, a fumed metal oxide such as a fumed silica or alumina, and aplurality or network of interconnected voids, where the voids providethe desired optical haze. In some cases, the weight ratio of the fumedmetal oxide to the binder is in a range from about 2:1 to about 6:1, orin a range from about 2:1 to about 4:1.

In some cases, optical film 965 can be or include any optical film thatincludes a plurality of voids, where the voids provide sufficientoptical haze and the optical film is sufficiently porous to promote ormaintain TIR or enhance internal reflection.

In the exemplary optical constructions 900 and 901, optical films 960and 965 fill the grooves in between optical elements 950 andsubstantially planarize retroreflecting structured surface 940, meaningthat top surfaces 923 and 959 of respective optical films 960 and 965are substantially planar. For example, in such cases, the differencebetween the maximum and minimum heights of top surface 923 as measuredfrom a common reference plane such as reference surface 936, is not morethan about 20%, or not more than about 15%, or not more than about 10%,or not more than about 5% of height h₁ of optical elements 950, where h₁is the distance 958 between apex 954 and base 956.

In some cases, the optical film does not planarize structured surface940. For example, FIG. 3 is a schematic side-view of an opticalconstruction 300 that includes an optical film 1060 that issubstantially conformally disposed on retroreflecting structured surface940 of retroreflecting layer 930, and an optical layer 310 that isdisposed on and substantially planarizes the optical film. Optical layer310 can, for example be a substrate similar to substrate 980, or anoptical adhesive layer similar to layer 970, or an optically diffusivelayer similar to layer 995.

Optical film 1060 has a thickness t₁. In some cases, thickness t₁ is notless than the minimum thickness required to substantially support totalinternal reflection at the retroreflecting surface 934. In such cases,t₁ is sufficiently large so that the evanescent tail of an incidentoptical ray 1090 at the interface between retroreflecting layer 930 andoptical film 1060 remains substantially within the optical film and doesnot extend, or extends very little, into the neighboring optical layer310. In such cases, incident light ray 1090 is totally internallyreflected as light ray 1092 and no fraction, or a very small fraction,of the incident light ray couples into layer 310. In such cases,thickness t₁ is not less than about 0.5 microns, or not less than about0.6 microns, or not less than about 0.7 microns, or not less than about0.8 microns, or not less than about 0.9 microns, or not less than about1 micron, or not less than about 1.1 microns, or not less than about 1.2microns, or not less than about 1.3 microns, or not less than about 1.4microns, or not less than about 1.5 microns, or not less than about 1.7microns, or not less than about 2 microns.

Optical film 1060 includes two structured major surfaces. In particular,the optical film comprises a first structured major bottom surface 1064that faces retroreflecting layer 930 and a second structured major topsurface 1062 that is opposite first structured major surface 1064.

Referring back to FIG. 1A, one of or both substrates 920 and 980 canprovide support to and increase the strength of retroreflecting opticalconstruction 900. Substrate 920 is substantially optically transmissive.For example, the optical transmittance of substrate 920 is not less thanabout 50%, or not less than about 60%, or not less than about 70%, ornot less than about 80%, or not less than about 90%.

Substrate 980 can be optically opaque or transmissive. In some cases,rear substrate 980 can be a rigid plate, such as a rigid aluminum plate.For example, optical construction 900 can be part of a road sign or amotor vehicle's license plate and substrate 980 can be a rigid aluminumback plate. In some cases, optical construction 900 does not include thefirst substrate 980.

The exemplary optical elements 950 in FIGS. 1A and 1B have pyramidal ortetrahedral shapes. In general, optical elements 950 can have anysuitable shape that can provide efficient reflection or retroreflectionin an application.

In some cases, such as when retroreflecting optical construction 900 isintended to be substantially flexible, retroreflecting layer 930 doesnot include land portion 932. In such cases, the retroreflecting opticalelements are not connected to each other through a land portion and canbe directly formed on, for example, substrate 920. The use of discreteunconnected cube-corner optical elements 950 can increase theflexibility of retroreflecting optical construction 900 because eachcube-corner optical element 950 can move independently of the othercube-corner optical elements.

Graphics layer 910 is an optional layer and includes one or more graphicimages for viewing by viewer 905 under suitable lighting conditions,such as daytime lighting conditions. A graphic image can be a coloredimage and can be optically transmissive for all the colors included inthe image, although the graphics layer can be more opticallytransmissive for brighter colors and less optically transmissive fordarker colors. In some cases, the optical transmittance of the graphicslayer for any color included in the layer is at least 5%, or at least7%, or at least 10%. Graphics layer 910 can be formed by any suitablemethod, such as any suitable printing or coating method, and can includedifferent colorants, such as different dyes or pigments, appropriatelydispersed in a binder.

In the exemplary retroreflecting optical construction 900, graphicslayer 910 is disposed on the front of the construction. In general, thegraphics layer, if included, can be disposed any where that may bedesirable in an application. For example, in some cases, the graphicslayer can be disposed between layers 920 and 930.

In the exemplary optical constructions 900 and 901, optical films 960and 965 cover substantially the entire structured retroreflectingsurface 940. In some cases, the optical films can be patterned and onlycover certain portions of surface 940. For example, FIG. 4 is aschematic side-view of an optical construction 400 that includes apatterned optical film 420 that only covers portions of surface 940. Inparticular, optical film 420 covers, substantially conformally, portions430 of surface 940, but does not cover and leaves exposed other portions432 of surface 940. Optical film 420 forms a pattern on surface 940. Insome cases, the pattern can be a regular pattern. In some cases, thepattern can be an irregular, such as a random, pattern. Optical film 420promotes TIR or enhances internal reflection and can be similar tooptical film 960 or 965.

Optical construction 400 also includes an optically diffusive layer 410that is disposed on optical film 420 and uncovered portions 430. In somecases, optically diffusive layer 410 includes a plurality of particles,such as a plurality of TiO₂ particles, dispersed in a binder, where theindex of the binder can be close to the index of refraction ofretroreflecting layer 930. In such cases, optical construction caneffectively retroreflect light in the covered portions 430, but not inthe uncovered portions 432. Optically diffusive layer 410 can give theoptical construction a white appearance in certain lighting, such as daylight, conditions.

FIG. 5 is a schematic side-view of an optical construction 500 thatincludes a patterned optical film 520 that only covers portions ofsurface 940. In particular, optical film 520 covers and substantiallyplanarizes portions 430 of surface 940, but does not cover and leavesexposed other portions 432 of surface 940. Optical construction 500 issimilar to optical construction 400 except that optical film 420conformally covers portions 430 of surface 940, whereas optical film 520planarizes portions 430 of surface 940.

Optical films 420 and 520 can be similar to any optical films disclosedherein. For example, optical films 420 and 520 can be similar to topoptical films 960 or 965. In some cases, the percent area of portions430 of retroreflecting surface 940 that are covered by optical film 420or 520, is less than about 50%, or less than about 40%, or less thanabout 30% of the total structured area.

Particle volume concentration (PVC) and critical particle volumeconcentration (CPVC) can be used to characterize the porosity of acoating. When the volume concentration of the particles is larger thanCPVC, the coating is porous since there is not enough binder to fill allthe gaps between the particles and the interstitial regions of thecoating. The coating then becomes a mixture of binder, particles andvoids. The volume concentration at which this occurs is related toparticle size and particle structure and/or shape. Formulations withvolume concentrations above CPVC have a volume deficiency of resin inthe mixture that is replaced by air. The relationship between CPVC, PVCand porosity is:

${Porosity} = {1 - \frac{CPVC}{PVC}}$

In some cases, the desirable values of CPVC for retroreflectiveconstructions are not greater than about 60%, or not greater than about50%, or not greater than about 40%. Particles that are highly branchedor structured prevent efficient packing in the binder matrix and allowinterstitial voids or pores to form. Exemplary structured and branchedmaterials are Cabo-Sil™ fumed silicas such as EH5, TS 520, PG 002, PG022, fumed alumina oxides such as PG003, and dispersible carbon blackssuch as those available from Cabot under the trade name Vulcan™ XC72R.

The surface area, porosity and skeletal density of a nanoporous coatingformulation can be determined by Braunauer, Emmett and Teller surfacearea analysis (the BET method). The porosity values obtained by BET canbe used to determine the CPVC. The BET method is a well-known method fordetermining pore size, surface area and percent porosity of a solidsubstance.

In some cases, the desirable BET porosities for the coating of some ofthe disclosed optical films are in a range from about 55% to about 80%,or in a range from about 60% to about 80%, or in a range from about 65to about 80%.

Referring back to FIG. 1A, retroreflecting optical construction 900includes retroreflecting layer 930 that includes retroreflectingstructured major surface 940. The optical construction also includesoptical film 960 that is generally disposed on a first portion of theretroreflecting structured major surface. For example, the first portioncan be portions 430 in FIG. 4. In some cases, the first portion is notless than about 30% of the retroreflecting structured major surface, ornot less than about 35% of the retroreflecting structured major surface,or not less than about 40% of the retroreflecting structured majorsurface, or not less than about 45% of the retroreflecting structuredmajor surface, or not less than about 50% of the retroreflectingstructured major surface, or not less than about 55% of theretroreflecting structured major surface, or not less than about 60% ofthe retroreflecting structured major surface, or not less than about 65%of the retroreflecting structured major surface, or not less than about70% of the retroreflecting structured major surface, or not less thanabout 75% of the retroreflecting structured major surface, or not lessthan about 80% of the retroreflecting structured major surface.

Optical film 960 supports TIR in the first portion of theretroreflecting structured major surface. In some cases, the firstportion of the retroreflecting structured major surface exhibits acoefficient of retroreflection R_(A) that is not less than about 50cd/(lux·m²), or not less than about 100 cd/(lux·m²), or not less thanabout 150 cd/(lux·m²), or not less than about 200 cd/(lux·m²), or notless than about 250 cd/(lux·m²), or not less than about 300 cd/(lux·m²),or not less than about 350 cd/(lux·m²), or not less than about 400cd/(lux·m²), for an observation angle of about 0.2 degrees and anentrance angle of about −4 degrees.

Total light return (TLR) for the retroreflecting optical construction900 can be determined from a knowledge of percent active area and rayintensity. Ray intensity can be reduced by front surface losses and byreflection from each of the three cube corner surfaces for aretroreflected ray. Total light return is defined as the product ofpercent active area and ray intensity, or a percentage of the totalincident light which is retroreflected. A discussion of total lightreturn for directly machined cube corner arrays is described in, forexample, U.S. Pat. No. 3,712,706 (Stamm). The total light return isfurther described in Provisional U.S. Patent Application No. 61/107,586,filed Oct. 22, 2008 incorporated herein by reference in its entirety.

In some cases, the first portion of the retroreflecting structured majorsurface exhibits a total light return that is not less than about 5%, ornot less than about 10%, or not less than about 15%, or not less thanabout 20%, or not less than about 25%, or not less than about 30%, forincident visible light at an entrance angle of about −4 degrees.

Some of the advantages of the disclosed films, layers, constructions,and systems are further illustrated by the following examples. Theparticular materials, amounts and dimensions recited in this example, aswell as other conditions and details, should not be construed to undulylimit the present invention.

EXAMPLE 1

Coating solutions 1-9 were made using hydrophobic resins listed in TableI. For each coating solution, the resin and the fumed silica (availableas TS-530 from Cabot Corporation, Billerica Mass.) at the weight ratiospecified in Table I were mixed with the corresponding solvent alsospecified in Table I. The resin had a wt-part of 1. For example, forcoating solution 1, the weight ratio of resin FC2145 to fumed silica was1:5.

The resin used in coating solutions 1, 2, and 9 was DyneonFluoroelastomer Copolymer FC2145 (available from Dyneon LLC, OakdaleMinn.). The resin used in coating solutions 3 and 4 was SPU-5k which wasa silicone polyurea formed from the reaction between an

ω aminopropyl polydiemthyl siloxane and m-tetramethyl xylene diisocyanteas generally described in U.S. Pat. No. 6,355,759, Example #23. Theresin used in coating solutions 5 and 6 was SR-351, a UV-polymerizablemonomer (available from Sartomer Company, Exton Pa.). The resin used incoating solutions 7 and 8 was Ebecryl 8807 (EB-8807), a UV-polymerizablemonomer (available from Cytec Corporation, West Paterson N.J.). Samples5-8 were UV curable and included 1% by weight of Esacure KB-1photoinitiator in methylethyl ketone (available from Lamberti USA,Conshohocken Pa.).

For each coating solution, the solvent was either isopropyl alcohol(IPA) or methanol (MeOH). The mixing of the resin, the fumed silica, andthe solvent was done in a 300 mL stainless steel beaker. The fumedsilica was dispersed in the resin using a Ross 100-LC single stage highshear mixer with a single stage slotted head rotor (available fromCharles Ross and Sons, Hauppauge N.Y.) for about 3 minutes at 1200 rpm.Next, the resulting foam was allowed to settle. Next, the solid weightpercentage was adjusted to 12% by adding more of the same solventresulting in coating solutions 1-9.

Next, a coating method was developed for each coating solution. First,the coating solution was coated on a PVC Vinyl organosol substrate(available as Geon 178 from PolyOne, Avon Lake Ohio) using a roundwire-rod (available as a Meyer rod from RD Specialties, Webster N.Y.),where the size of the rod is specified in Table I. The wet coatingthickness was dictated by the wire-rod number. A number 30 wire-rodresulted in a wet coating thickness of approximately 75.2 microns, and anumber 15 wire-rod resulted in a wet coating thickness of approximately38.1 microns.

Coated samples 1-4 and 9 were dried at room temperature for 25 minutes.Coated samples 5-8 were cured with UV radiation using a Fusion SystemsLight Hammer UV system (available from Fusion Systems Inc, Gaithersburg,Md.) that was equipped with a 500 Watt H-bulb. The coatings were curedwith a single exposure at 40 feet per minute (12.3 meters per minute)which corresponded to a UV-B dose of about 49 mille-joules per squarecm.

TABLE I Formulation and coating parameters for Example 1 Coating Resinf-SiO₂ Coating Photo Solution # (wt-part = 1) (wt-part) Solvent RodInitiator 1 FC2145 5 MeOH 30 — 2 FC2145 5 MeOH 15 — 3 SPU-5k 5 IPA 30 —4 SPU-5k 5 IPA 15 — 5 SR-351 5 IPA 30 1% KB-1 6 SR-351 5 IPA 15 1% KB-17 EB-8807 5 IPA 30 1% KB-1 8 EB-8807 5 IPA 15 1% KB-1 9 FC2145 0 MeOH 30—

EXAMPLE 2

Coating solutions 10-15 were made using hydrophilic polyvinylalcohol(available as Poval PVA-235 from Kuraray America, Houston Tex.) asspecified in Table II. For each coating solution, the resin and thefumed silica (available as Cabo-O-Sperse PG002 from Cabot Corporation,Billerica Mass.) were mixed at the weight ratio specified in Table II.The resin had a wt-part of 1. For example, for coating solution 10, theweight ratio of the resin, PVA-235, to the fumed silica was 1:4. First,the PVA-235 resin was added as a 7% by weight solution in water to astainless steel beaker equipped with an air drive mixer that wasoperated at a low speed to minimize foaming. Tergitol Min-Foam XL(available from Dow Chemical Company, Midland Mich.) at 1% of the weightof PVA-235, and NH₄OH at 2-3% of the weight of PVA-235, were added tothe mixer to adjust the pH to approximately 9.5-10. Next, fumed silicawas added as a 20% by weight solution in water. If needed, a sufficientquantity of CX-100, specified in Table II as a percent of the weight ofthe resin, was added to the mixer and the mixture was stirred until ahomogenized solution was obtained. Next, deionized water was added toadjust the percentage of solids to the value specified in Table II.

Next, a coating method was developed for each coating solution. Anautomated notch bar coating process was used to coat each coatingsolution on a PVC Vinyl organosol substrate at a coating speed of 7.62meters per second. The resulting coated sample was then dried at 65° C.for 5 minutes.

TABLE II Formulation and coating parameters for Example 2 Coating PG-002Wt % Final wt % Solution # Resin wt-part CX100 Solids 10 PVA-235 4 013.5 11 PVA-235 4 10 13.5 12 PVA-235 6 0 15 13 PVA-235 6 10 15 14PVA-235 6 15 15 15 PVA-235 0 0 15

EXAMPLE 3

Retroreflecting optical construction 3000, a schematic side-view ofwhich is shown in FIG. 6, was made. Optical construction 3000 wassimilar to a corresponding construction in FIG. 1A and included flexibleprismatic retroreflecting layer 930 and optical film 960 coated on theretroreflecting layer. The optical film substantially planarized thestructured side of the retroreflecting layer. FIGS. 7A and 7B arerespective schematic top-view and side-view of an individual prism inthe prismatic retroreflecting layer. The angles in FIGS. 7A and 7B arein degrees, and the dimensions are in mils. Each facet of a prism in theretroreflecting layer was a right angled triangle and the base was anisosceles triangle. The prisms were made using the methods generallydescribed in, for example, U.S. Pat. Nos. 6,843,571 and 5,691,846, thedisclosures of which are incorporated in their entireties herein byreference.

Retroreflecting optical constructions 1-9, similar to construction 3000,were made following the procedure described in Example 2. Constructions1-9 corresponded to respective samples 1-9 made in Example 2.Construction “A” was the prismatic retroreflecting layer with no coating(that is, layer 960 was air).

Coefficient of retroreflection R_(A) in units of cd/(lux·m²) weremeasured for the constructions according to ASTM E-810 test method at0.2 degree observation angle and −4 degrees entrance angle, and at 0.2degree observation angle and 30 degrees entrance angle. The measurementswere taken for two orthogonal prism orientations. The measured resultsfor R_(A) are summarized in Table III.

TABLE III Retroreflective properties of samples in Example 3 ObservationObservation Angle: 0.2° Angle: 0.2° Entrance Entrance Angle: −4° Angle:30° Film Orientation Construction (Degrees) No. 0 90 0 90 1 363 355 5457 2 304 287 53 33 3 199 200 20 27 4 294 273 32 23 5 234 214 30 25 6 174160 20 18 7 272 256 37 30 8 273 254 41 28 9 0.1 0.1 0.3 0.4 A 571 574 6952

EXAMPLE 4

Retroreflecting optical constructions 10-15, similar to construction3000, were made following the procedure described in Example 2.Constructions 10-15 corresponded to respective samples 10-15 made inExample 2. Construction “B” was the prismatic retroreflecting layer withno coating (that is, layer 960 was air).

Coefficient of retroreflection R_(A) in units of cd/(lux·m²) weremeasured for the optical constructions at 0.2 degree observation angleand −4 degrees entrance angle, and at 0.2 degree observation angle and40 degrees entrance angle. The measurements were taken for twoorthogonal prism orientations. The measured results for R_(A) aresummarized in Table IV.

TABLE IV Retroreflective properties of samples in Example 4 ObservationObservation Angle: 0.2° Angle: 0.2° Entrance Entrance Angle: −4° Angle:40° Film Orientation Construction (degrees) No. 0 90 0 90 10 324 32028.1 14.2 11 322 318 22.8 15 12 363 356 30 16.8 13 336 335 21.5 18.9 14297 283 11.6 11.5 15 0.1 0.1 0.1 0.1 B 571 574 29 17

EXAMPLE 5

Retroreflecting optical constructions 3000 were made. Prismaticretroreflecting layer 930 was rigid and made using the methods generallydescribed in, for example, U.S. Pat. No. 6,884,371, the disclosure ofwhich is incorporated in its entirety herein by reference. The opticalfilm substantially planarized the structured side of the retroreflectinglayer. FIGS. 8A and 8B are respective schematic top-view and side-viewof an individual prism in the prismatic retroreflecting layer. Theangles in FIGS. 8A and 8B are in degrees, and the dimensions are inmils. The facets of the prisms were right angled triangles and the baseswere isosceles triangles.

Retroreflecting optical constructions 1-7, similar to construction 3000,were made following the procedure described in Example 1. Constructions1-6 corresponded to respective samples 1-6 made in Example 1. Opticalconstruction 7 corresponded to sample 9 in Example 1. Construction “C”was the prismatic retroreflecting layer with no coating (that is, layer960 was air).

Coefficient of retroreflection R_(A) in units of cd/(lux·m²) weremeasured at 0.2 degree observation angle and −4 degrees entrance angle,and at 0.2 degree observation angle and 30 degrees entrance angle. Themeasurements were taken for two orthogonal prism orientations. Themeasured results for R_(A) are summarized in Table V.

TABLE V Retroreflective properties of samples in Example 5 ObservationObservation Angle: 0.2° Angle: 0.2° Entrance Entrance Angle: −4° Angle:30° Film Orientation Construction (Degrees) No. 0 90 0 90 1 520 587 136208 2 1092 1180 380 426 3 1220 1260 399 467 4 1228 1244 399 424 5 448610 102 204 6 315 338 62 129 7 15 19 30 30 C 1260 903 644 605

EXAMPLE 6

Retroreflecting optical construction 3000, a schematic side-view ofwhich is shown in FIG. 6, was made. Optical construction 3000 wassimilar to a corresponding construction in FIG. 1A and included flexibleprismatic retroreflecting layer 930 and optical film 960 coated on theretroreflecting layer. The optical film substantially planarized thestructured side of the retroreflecting layer. FIGS. 7A and 7B arerespective schematic top-view and side-view of an individual prism inthe prismatic retroreflecting layer. The angles in FIGS. 7A and 7B arein degrees, and the dimensions are in mils. Each facet of a prism in theretroreflecting layer was a right angled triangle and the base was anisosceles triangle. The prisms were made using the methods generallydescribed in, for example, U.S. Pat. Nos. 6,843,571 and 5,691,846.

First, a coating solution was made. In a 2 liter three-neck flask,equipped with a condenser and a thermometer, 960 grams of IPA-ST-UPorganosilica elongated particles (available from Nissan Chemical Inc.,Houston, Tex.), 19.2 grams of deionized water, and 350 grams of1-methoxy-2-propanol were mixed under rapid stirring. The elongatedparticles had a diameter in a range from about 9 nm to about 15 nm and alength in a range from about 40 nm to about 100 nm. The particles weredispersed in a 15.2% wt IPA. Next, 22.8 grams of Silquest A-174 silane(available from GE Advanced Materials, Wilton, Conn.) was added to theflask. The resulting mixture was stirred for 30 minutes.

The mixture was then kept at 81° C. for 16 hours. Next, the solution wasallowed to cool down to room temperature. Next, about 950 grams of thesolvent in the solution were removed using a rotary evaporator under a40° C. water-bath, resulting in a 41.7% wt A-174-modified elongatedsilica clear dispersion in 1-methoxy-2-propanol.

Next, 407 grams of this clear dispersion, 165.7 grams of SR 444(available from Sartomer Company, Exton, Pa.), 8.28 grams ofphotoinitiator Irgacure 184 and 0.828 grams of photoinitiator Irgacure819 (both available from Ciba Specialty Chemicals Company, High PointN.C.), and 258.6 grams of isopropyl alcohol were mixed together andstirred resulting in a homogenous coating solution of 40% solids.

Next, a coating method was developed for the coating solution.Approximately 1 ml of the 40% solids coating solution was applied to theflexible prismatic retroreflecting layer. A 1.0 mil thick SBOPP(simultaneously biaxially oriented polypropylene) liner was handlaminated onto the solution to create a uniform layer of coatingsolution. The liner was slightly above the peaks of the corner cubes.Next, the sample was cured in a single exposure by passing the samplethrough a belt-fed ultra-violet lamp system (available from RPCindustries, Plainfield, Ill.) fitted with two 200 Watt medium pressureHg bulbs, at 50 fpm, yielding a UVA dose of 300 mJ/cm² in air. Thesamples were then removed from the chamber, the SBOPP liner was removed,and the sample was placed in a 120° F. oven for about 10 minutes to dry.

Coefficient of retroreflection R_(A) in units of cd/(lux·m²) weremeasured for the constructions according to ASTM E-810 test method at0.2 degree observation angle and −4 degrees entrance angle, and at 0.2degree observation angle and 30 degrees entrance angle. The measurementswere taken for two orthogonal prism orientations. The measured resultsfor R_(A) are summarized in Table VI.

TABLE VI Retroreflective properties of samples in Example 6 ObservationObservation Angle: 0.2° Angle: 0.2° Entrance Entrance Angle: −4° Angle:30° Film Orientation Construction (Degrees) No. 0 90 0 90 Example 6 99.6101 6 4

EXAMPLE 7

A 25 g solution of the coating formulation 12 described in Example 2 wasdried at 50° C. in a 200 ml beaker. The dried formulation was collectedand ground into a fine powder with a ceramic motor and pestle and wasdried further at 80° C. for 16 hrs. The solid powder was then submittedfor BET analysis along with control samples CE-A, CE-B and CE-C preparedin a similar manner. Control sample CE-A was made usingPoly(methylmethacrylate)-Cabot TS 530 f-SiO₂ mixture (PMMA-Si 1:5, wherethe PMMA was obtained from Aldrich Chemicals and the mixture was driedfrom 15% solids in MEK instead of water). Control sample CE-B was madefrom a PMMA-NALCO 2327 1:5 by weight mixture (Nalco 2327 was anon-porous 20 nm colloidal silica dispersion available from Rohm andHaas of Philadelphia, Pa.). Control sample CE-C was made from Cabot TS530 f-SiO₂ with no resin. The BET data are shown in Table VII.

The surface area, porosity and skeletal density of the dried coatingformulations were measured by means of a Quantachrome Autosorb 1 BETanalyzer (available from Quantachrome Instruments of Boynton Beach.Fla.). The samples were subjected to a 40 point analysis to determinetheir surface area and pore size distribution. The BET method of surfacearea analysis (due to Braunauer, Emmett and Teller) was used todetermine pore size, surface area and percent for each of the samples.

TABLE VII Pore volume, pore fraction and surface area for samples ofExample 7 Pore Pore Volume Fraction Surface Sample Mixture (cc/g) (NLDF)Area m²/g 12 1:6 PVA-Si 0.86 63% 107 CE-A 1:5 PMMA-Si 0.953 65% 118 CE-B1:5 PMMA- 0.098 16% 11 collodial silica (non-porous silica)

As used herein, terms such as “vertical”, “horizontal”, “above”,“below”, “left”, “right”, “upper” and “lower”, “clockwise” and “counterclockwise” and other similar terms, refer to relative positions as shownin the figures. In general, a physical embodiment can have a differentorientation, and in that case, the terms are intended to refer torelative positions modified to the actual orientation of the device. Forexample, even if optical construction 900 in FIG. 1A is flipped ascompared to the orientation in the figure, major surface 923 is stillconsidered to be a “top” major surface.

All patents, patent applications, and other publications cited above areincorporated by reference into this document as if reproduced in full.While specific examples of the invention are described in detail aboveto facilitate explanation of various aspects of the invention, it shouldbe understood that the intention is not to limit the invention to thespecifics of the examples. Rather, the intention is to cover allmodifications, embodiments, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

What is claimed is:
 1. A retroreflecting optical constructioncomprising: a retroreflecting layer having a retroreflecting structuredmajor surface; and an optical film disposed on the retroreflectingstructured major surface of the retroreflecting layer and having anoptical haze that is not less than about 30%, wherein substantialportions of each two neighboring major surfaces in the retroreflectingoptical construction are in physical contact with each other.
 2. Theretroreflecting optical construction of claim 1, wherein structures inthe retroreflecting structured major surface comprise a plurality ofpyramidal structures.
 3. The retroreflecting optical construction ofclaim 2, wherein the plurality of pyramidal structures comprises aplurality of cube corners.
 4. The retroreflecting optical constructionof claim 1, wherein at least 50% of each two neighboring major surfacesin the retroreflecting optical construction are in physical contact witheach other.
 5. The retroreflecting optical construction of claim 1,wherein the optical film comprises a plurality of particles and aplurality of interconnected voids, wherein a volume fraction of theplurality of interconnected voids in the optical film is not less thanabout 20%, and wherein a weight ratio of the plurality of the particlesto the binder is in a range from about 2:1 to about 6:1.
 6. Theretroreflecting optical construction of claim 5, wherein the volumefraction of the plurality of interconnected voids in the optical film isnot less than about 50%.
 7. The retroreflecting optical construction ofclaim 5, wherein the weight ratio of the plurality of the particles tothe binder is in a range from about 2:1 to about 4:1.
 8. Aretroreflecting optical construction comprising: a retroreflecting layerhaving a retroreflecting structured major surface; and an optical filmdisposed on a first portion of the retroreflecting structured majorsurface and comprising a binder, a plurality of particles and aplurality of voids, wherein the first portion of the retroreflectingstructured major surface exhibits a coefficient of retroreflection R_(A)that is not less than about 50 cd/(lux·m²) for an observation angle of0.2 degrees and an entrance angle of −4 degrees.
 9. The retroreflectingoptical construction of claim 8, wherein the first portion is not lessthan about 30% of the retroreflecting structured major surface.
 10. Theretroreflecting optical construction of claim 8, wherein R_(A) is notless than about 100 cd/(lux·m²) for an observation angle of 0.2 degreesand an entrance angle of −4 degrees.
 11. A retroreflecting opticalconstruction comprising: a retroreflecting layer having aretroreflecting structured major surface; and an optical film disposedon a first portion of the retroreflecting structured major surface andcomprising a binder, a plurality of particles and a plurality of voids,wherein the first portion of the retroreflecting structured majorsurface exhibits a total light return that is not less than about 5% forincident visible light at an entrance angle of −4 degrees.
 12. Theretroreflecting optical construction of claim 11, wherein the totallight return is not less than about 10% for incident visible light at anentrance angle of −4 degrees.